Ball-and-roller bearing and method of manufacturing the same

ABSTRACT

A ball-and-roller bearing is disclosed having an inner ring, an outer ring arranged on the axis of the inner ring and rotating around the co-axis relative to the inner ring, and a rolling body interposed between the inner ring and the outer ring and rolling with rotation of the outer ring relative to the inner ring. One of the inner ring, the outer ring, and the rolling body has a core member made from an alloy containing iron, at least one of 0.2 to 1.0% by weight of silicon and 0.2 to 1.5% by weight of manganese, 7.0 to 11.0% by weight of chromium, 1.5 to 6.0% by weight of molybdenum, and 0.5 to 8.0% by weight of cobalt, and a case hardened surface layer. Also disclosed is the method of manufacturing the bearing by forming a core member of the alloy, forming a case hardened layer containing 0.9 to 1.5% by weight of carbon in a surface region of the core member by applying to the core member a carbonizing or carbonitriding treatment, a hardening treatment, and a tempering treatment under high temperatures, and assembling the inner ring, the outer ring and the rolling body such that the inner ring and the outer ring are positioned on the same axis and the rolling body is between the inner ring and the outer ring.

TECHNICAL FIELD

The present invention relates to a ball-and-roller bearing and a methodof manufacturing the same, particularly to a ball-and-roller bearingused in, for example, an aircraft and a method of manufacturing thesame.

BACKGROUND ART

A high mechanical strength is required for a ball-and-roller bearingused under conditions of a high temperature and a high rotating speedlike the ball-and-roller bearing used in an engine of an aircraft.Particularly, since only a slight damage done to a member is likely tobring about a serious accident in an aircraft, a very high mechanicalstrength and reliability is required for the ball-and-roller bearing foran aircraft.

For the same reasons, the ball-and-roller bearing for an aircraft isjudged in general to have reached the life at the time when only aslight rust has been formed. Such being the situation, theball-and-roller bearing for an aircraft is required to exhibit not onlya high mechanical strength but also an excellent corrosion resistance.

It was customary to use a semihice series materials of AISI M50, M50NiL,etc. for forming the ball-and-roller bearing for an aircraft. Theball-and-roller bearing for an aircraft that is made of these materialsexhibits a relatively high mechanical strength. However, in thesematerials, a content of chromium that is said to be most effective forimproving the corrosion resistance of the steel is low.

In general, an airport is constructed near the coast, since the airportrequires a tremendously large site and serious noises are generated bythe aircraft around the airport. What should be noted is that theball-and-roller bearing for an aircraft is used or stored under theenvironment in which rust tends to be caused by salt.

In the ball-and-roller bearing for an aircraft that is made of thesemihice series material, the problem derived from the rust is not soserious in respect of the rolling body. This is because the rolling bodyhas a small surface area, compared with other members. In addition, themost portion of the surface is brought into a rolling or sliding contactwith other members. However, each of the inner ring and the outer ringhas a large surface area, and the most portion of the surface is notbrought into a rolling or sliding contact with other members. Inaddition, the ball-and-roller bearing for an aircraft including an innerring or an outer ring made of the material described above fails toexhibit a sufficiently high corrosion resistance.

A martensite series stainless steel such as SUS440C is known to besuitable for use as a material of the ball-and-roller bearing excellentin corrosion resistance. However, in the case of using a martensiteseries stainless steel for forming the ball-and-roller bearing for anaircraft, coarse eutectic carbide particles are formed in the innerring, outer ring, etc. As a result, it is not possible to obtain ahardness high enough to be used satisfactorily under the particularlysevere conditions of a high temperature and a high rotating speed. Underthe circumstances, it is important to improve further the rollingfatigue life characteristics of the ball-and-roller bearing.

DISCLOSURE OF INVENTION

The present invention, which has been achieved in view of theabove-noted problems inherent in the prior art, is intended to provide aball-and-roller bearing excellent in mechanical characteristics andcorrosion resistance and a method of manufacturing the same.

According to the present invention, there is provided a ball-and-rollerbearing, comprising an inner ring, an outer ring arranged on the co-axisof the inner ring and rotating around the axis relative to the innerring, and a rolling body interposed between the inner ring and the outerring and rolling on the inner ring and the outer ring in accordance withrotation of the outer ring relative to the inner ring, wherein at leastone member selected from the group consisting of the inner ring, theouter ring, and the rolling body comprises a core member consistingessentially of an alloy containing iron, at least one of 0.2 to 1.0% byweight of silicon and 0.2 to 1.5% by weight of manganese, 7.0 to 11.0%by weight of chromium, 1.5 to 6.0% by weight of molybdenum, and 0.5 to8.0% by weight of cobalt, and a case hardened layer formed by subjectinga surface region of the core member to a secondary hardening treatmentand containing 0.9 to 1.5% by weight of carbon.

The present invention also provides a method of manufacturing aball-and-roller bearing, comprising the steps of forming a core memberconsisting essentially of an alloy containing iron, at least one of 0.2to 1.0% by weight of silicon and 0.2 to 1.5% by weight of manganese, 7.0to 11.0% by weight of chromium, 1.5 to 6.0% by weight of molybdenum, and0.5 to 8.0% by weight of cobalt, forming a case hardened layercontaining 0.9 to 1.5% by weight of carbon in a surface region of thecore member by applying to the core member a carborizing orcarbonitriding treatment, a hardening treatment, and a temperingtreatment under high temperatures in the order mentioned, and assemblingan inner ring, an outer ring and a rolling body such that the inner ringand the outer ring are positioned on the same axis and that the rollingbody is interposed between the inner ring and the outer ring, at leastone of the inner ring, the outer ring, and the rolling body being formedof the core member having the case hardened layer formed on the surface.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross sectional view schematically showing a ball-and-rollerbearing according to one embodiment of the present invention;

FIG. 2 is a cross sectional view schematically showing a ball-and-rollerbearing according to another embodiment of the present invention;

FIG. 3 is a graph showing the conditions of the heat treatment appliedto ball-and-roller bearings according to an example of the presentinvention and a comparative example;

FIG. 4 is a cross sectional view schematically showing a thrust typelife tester used for measuring the rolling fatigue life of aball-and-roller bearing according to the example of the presentinvention;

FIG. 5 is a graph showing the relationship between the cobaltconcentration of steel in a case hardened bearing steel and the surfacehardness;

FIG. 6 is a graph showing the relationship between the molybdenumconcentration in the case hardened bearing steel and the surfacehardness; and

FIG. 7 is a graph showing the relationship among the molybdenumconcentration and the cobalt concentration in the case hardened bearingsteel and the surface hardness.

BEST MODE OF CARRYING OUT THE INVENTION

A ball-and-roller bearing of the present invention will now be describedin detail with reference to the accompanying drawings.

FIG. 1 is a cross sectional view schematically showing a ball-and-rollerbearing according to one embodiment of the present invention. Also, FIG.2 is a cross sectional view schematically showing a ball-and-rollerbearing according to another embodiment of the present invention. Aball-and-roller bearing 1 shown in each of FIGS. 1 and 2 is constructedmainly of an inner ring 2, an outer ring 3 arranged on the axis of theinner ring 2, and a rolling body 4 interposed between the inner ring 2and the outer ring 3. Note that the reference numeral 5 denotes a holderfor holding the rolling body 4.

In general, a groove-like track is formed on each of the mutually facingsurfaces of the inner ring 2 and the outer ring 3. The rolling body 4 isrolled along the tracks in accordance with rotation of the inner ring 2relative to the outer ring 3.

The shapes of the inner ring 2 and the outer ring 3 included in theball-and-roller bearing 1 of the present invention are not particularlylimited. It is possible for these inner ring 2 and outer ring 3 to beshaped like the inner and outer rings used in the generalball-and-roller bearing. At least one of the inner ring 2 and the outerring 3 may be divided into two parts along a plane perpendicular to therotary axis, as shown in FIG. 1. It is possible for the inner ring 2 tobe formed integral with the shaft supported by the ball-and-rollerbearing 1. The rolling body 4 may be shaped like the rolling body usedin the ordinary ball-and-roller bearing. For example, the rolling body 4may be shaped like a ball as shown in FIG. 1 or like a roll as shown inFIG. 2.

In the ball-and-roller bearing 1 of the present invention, at least oneof the inner ring 2, the outer ring 3 and the rolling body 4 is formedby molding an alloy of a predetermined composition into a desired shape,followed by applying a cementating or carbonitriding treatment to themolded article and subsequently applying a hardening treatment and atempering treatment to the molded article in the order mentioned. As aresult, the surface region of the molded articles is secondary hardened,with the result that a case hardened layer is formed on the moldedarticle. On the other hand, the alloy composition is retained in thecore portion of the molded article. The ball-and-roller bearing 1 of thepresent invention is featured in that at least one of the inner ring 2,the outer 3 and the rolling body 4 is formed of a case hardened bearingsteel constituted by the core member consisting of the alloy describedabove and the case hardened layer formed in the surface region of thecore member. The case hardened bearing steel will now be described.

In the ball-and-roller bearing 1 of the present invention, the alloyused as a material of the case hardened bearing steel contains iron as amain component. In addition, at least one of silicon and manganese,cobalt, chromium and molybdenum are contained in the alloy together withiron.

Used in the ball-and-roller bearing 1 of the present invention is a casehardened bearing steel having a case hardened layer containing a highconcentration of carbon and a core portion in which the carbonconcentration is suppressed at a low level. In general, the carbonaddition to an alloy permits formation of a carbide so as to convert thetexture of the base material into martensite texture, thereby toincrease the mechanical strength of the base material. It follows that,in the case of using such a case hardened bearing steel, it is possibleto obtain a surface hardness high enough to withstand the severe rollingor sliding contact with another member. Also, where the carbonconcentration is suppressed at a low level in the core portion, a highmechanical strength (fracture toughness) can be obtained in the coreportion. It follows that the ball-and-roller bearing 1 of the presentinvention is capable of absorbing a strong impact, even if applied, and,thus, fracture is unlikely to take place.

A major portion of the carbon atoms or nitrogen atoms contained in thecase hardened layer is supplied by the cementating or carbonitridingtreatment applied to the molded article which is obtained by moldingalloy into a desired shape. It follows that the carbon concentration inthe core portion can be suppressed at a low level by using an alloyhaving a very low carbon concentration. In other words, it is possibleto increase the fracture toughness of the core portion.

Where the carbon content of the alloy is 0.2% by weight or less, a lowcarbon concentration can be retained in the core portion of the casehardened bearing steel, making it possible to suppress the hardness ofthe core portion at a sufficiently low level. Particularly, where thecarbon content of the alloy is 0.15% by weight or less, the hardness ofthe core portion can be suppressed at a very low level. It should benoted that the composition of the core portion of the molded articlebefore formation of the case hardened layer is left substantiallyunchanged after formation of the case hardened layer. In other words,the composition of the core portion is considered to be equal to thecomposition of the alloy used as a raw material of the case hardenedbearing steel.

The alloy used in the present invention contains at least one of 0.2 to1.0% by weight of silicon and 0.2 to 1.5% by weight of manganese.Silicon is used as a deoxidizer. A sufficient deoxidizing effect can beobtained, if silicon is added in an amount of about 0.2% by weight.However, if silicon is added in an amount exceeding 1.0% by weight, thecementating and forging properties of the alloy tend to be impaired.Manganese is also used as a deoxidizer. A sufficient deoxidizing effectcan be obtained, if manganese is added in an amount of about 0.2% byweight. Manganese also serves to improve the hardenability of the alloy.If manganese is added in an amount exceeding 1.5% by weight, however,the toughness of the case hardened bearing steel tends to be impaired.

The alloy used in the present invention also contains 7.0 to 11.0% byweight of chromium. As described previously, a high corrosion resistanceis required in the ball-and-roller bearing used in, for example, anaircraft. Chromium is most adapted for imparting a corrosion resistanceto steel. A sufficient corrosion resistance can be obtained by addingabout 7.0% by weight of chromium to the alloy. Particularly, a moresatisfactory corrosion resistance can be obtained in the case of adding8% by weight or more of chromium. It should also be noted that, wherechromium is contained in the alloy, carbides such as Cr₂₃C₆ and Cr₇C areformed in the hardening step so as to increase the hardness. Further,where the chromium concentration is set at 11% by weight or less,seizing is less likely to take place even in the case where the bearingis used under conditions of a high temperature and a high rotating speedlike the ball-and-roller bearing for an aircraft. Also, in this case,δ-ferrite is unlikely to be precipitated, making it possible to suppressreduction in the fracture toughness of the core portion. Particularly,where the chromium concentration is set at 10% by weight or less, theseizing occurrence and the reduction in the fracture toughness of thecore portion can be suppressed more effectively.

The alloy used in the present invention also contains 1.5 to 6.0% byweight of molybdenum and 0.5 to 8.0% by weight of cobalt. Each ofmolybdenum and cobalt is required for the secondary hardening. To bemore specific, molybdenum and cobalt form carbides or intermetalliccompounds in the surface region in the tempering step carried out athigh temperatures. The carbides precipitated in the tempering treatmentunder high temperatures have a chemical structure represented by ageneral formula “M₆C”, where M represents molybdenum and cobalt. Thegeneral formula indicates that the sum of the number of molybdenum atomsand the number of cobalt atoms is 6 times as large as the number ofcarbon atoms.

These carbides and the intermetallic compounds formed by molybdenum andcobalt form very fine crystal grains of submicron order. It followsthat, by adding molybdenum and cobalt to the alloy, the hardness of thecase hardened layer can be retained at a very high level even under hightemperatures. Also, where molybdenum is added to the alloy, thecorrosion resistance of the alloy can be improved. These effects can beobtained where the alloy contains 1.5% by weight or more of molybdenum.Also, these effects are rendered prominently high where the alloycontains 2.0% by weight or more of molybdenum. However, if molybdenum isadded in an amount exceeding 6.0% by weight, decarbonization anddemolybdenization tend to take place easily. Also, the toughness islikely to be lowered.

Cobalt also produces the effects described above. In addition, cobaltserves to prevent formation of δ-ferrite so as to obtain a hightoughness. Further, cobalt forms a solid solution with iron so as toincrease the amount of carbon contained in the alloy in the form of asolid solution. It follows that the hardness under high temperatures canbe increased. These effects can be obtained in the case of adding cobaltin an amount of 0.5% by weight or more and can be made more prominent inthe case where cobalt is contained in an amount of 2.0% by weight ormore. However, if the cobalt content of the alloy exceeds 8.0% byweight, the toughness of the alloy tends to be lowered.

It is desirable for the alloy used in the present invention to containvanadium in an amount of 0.1 to 1.0% by weight. Vanadium serves toincrease the softening resistance in the tempering step and to form acarbide having a chemical formula VC, which has a high hardness, in thehardening step. Where vanadium is added in an amount of 0.1% by weightor more, the hardness of the alloy under high temperatures is increasedso as to increase the abrasion resistance. However, if the vanadiumcontent of the alloy exceeds 1.0% by weight, the toughness of the alloytends to be lowered.

It is also desirable for the alloy used in the present invention tocontain nickel in an amount of 1.0 to 5.0% by weight. Where the alloycontains 1.0% by weight or more of nickel, δ-ferrite formation can beprevented so as to obtain a high toughness, as in the case of addingcobalt to the alloy. However, if nickel is added in an amount exceeding5.0% by weight, the A1 critical temperature is lowered so as to increasethe annealing hardness. It follows that the machinability of the alloytends to be lowered. It is more desirable for the nickel content of thealloy to be 3.0% by weight or less. In this case, the reduction in themachinability can be prevented satisfactorily.

The alloy used for forming the ball-and-roller bearing 1 of the presentinvention may also contain traces of impurities in addition to theadditive elements described above. These impurities include, forexample, phosphorus, sulfur and oxygen.

The case hardened bearing steel used in the ball-and-roller bearing 1 ofthe present invention can be obtained by applying a cementating orcarbonitriding treatment to the alloy described above, followed by ahardening treatment and subsequently a tempering treatment under hightemperatures. A case hardened layer exhibiting a high hardness evenunder high temperatures can be formed by these treatments, making itpossible to improve the rolling fatigue life characteristics.Incidentally, the term “case hardened layer” used herein represents asurface region of the member made of the case hardened bearing steel.The thickness of the case hardened layer is defined to be 2% of thediameter of the rolling body.

In the ball-and-roller bearing 1 of the present invention, at least oneof the inner ring 2, the outer ring 3 and the rolling body 4 is formedof the case hardened bearing steel described above. It follows that thebearing 1 of the present invention exhibits an excellent corrosionresistance and an excellent mechanical strength.

The inner ring 2, the outer ring 3 or the rolling body 4 consisting ofthe case hardened bearing steel can be manufactured, for example, asfollows. In the first step, a molded article of a predetermined shape isprepared by using the alloy described above. Then, a cementatingtreatment or a carbonitriding treatment, a hardening treatment, and atempering treatment under high temperatures are applied in the ordermentioned to the molded article. As a result, a case hardened layer isformed in a surface region of the molded article, with the compositionof the core portion of the molded article before these treatments leftunchanged. Then, a finish treatment such as grinding is applied, ifnecessary, to the molded article. Any of the inner ring 2, the outerring 3 and the rolling body 4 consisting of the case hardened bearingsteel can be manufactured in this fashion.

The inner ring 2 or the like thus manufactured has a case hardened layerin the surface region. What should be noted is that, in theball-and-roller bearing 1 of the present invention, those portions ofthe rolling body 4, the inner ring 2 and the outer ring 3 which arebrought into a rolling or sliding contact with other members of thebearing 1 are formed of the case hardened layers, with the result thatthe ball-and-roller bearing 1 of the present invention exhibits anexcellent mechanical strength. Further, the state of the core portionbefore formation of the case hardened layer is left unchanged afterformation of the case hardened layer, making it possible to achieve ahigh fracture toughness.

In the ball-and-roller bearing 1 of the present invention, the casehardened layer contains carbon. As described previously, where the alloycontains carbon, carbides are formed within the alloy together withother additives such as molybdenum, cobalt, chromium or vanadium so asto further increase the hardness of the alloy. Particularly, thecarbides of molybdenum and cobalt form very fine crystal grains ofsubmicron order, with the result that the hardness of the case hardenedlayer is kept at a very high level even if the bearing 1 is used undersevere conditions such as a high temperature and a high rotating speed.It should also be noted that, where the alloy contains carbon, some ofthe carbon atoms forms a solid solution within the alloy so as toincrease the hardness of the rolling and sliding contact surface.

In the ball-and-roller bearing 1 of the present invention, the carbonconcentration in the case hardened layer is controlled at 0.9 to 1.5% byweight. Where the carbon concentration in the case hardened layer is0.9% by weight or more, it is possible to form a case hardened layerhaving a sufficient hardness, making it possible to obtain a highabrasion resistance and satisfactory rolling fatigue lifecharacteristics even if the bearing 1 is used under severe conditionssuch as a high temperature and a high rotating speed. However, if thecarbon concentration in the case hardened layer exceeds 1.5% by weight,cementite that is an iron carbide is formed in the shape of coarsemeshes. The coarse cementite can be a start point of crack, with theresult that the rolling fatigue life of the bearing 1 tends to beshortened. The shortening of the rolling fatigue life can be preventedby setting the carbon concentration in the case hardened layer at 1.5%by weight or less. Particularly, where the carbon concentration in thecase hardened layer is set at 1.2% by weight or less, the shortening ofthe rolling fatigue life can be prevented more effectively.

Incidentally, if the carbon concentration in the case hardened layer isincreased, the corrosion resistance is lowered. However, the presentinventors have found that, if the carbon concentration in the casehardened layer is not higher than 1.5% by weight, it is possible toobtain a corrosion resistance as well as or better than that of thebearing prepared by using the conventional material of martensite seriesstainless steel.

As described previously, the case hardened layer is defined in thepresent invention to represent a surface region of the member made of acase hardened bearing steel and to have a thickness equal to 2% of thediameter of the rolling body. The thickness of the case hardened layeris defined to be 2% of the diameter of the rolling body because theprominent effects of the present invention described above can beobtained if the thickness of the surface region having a carbonconcentration of 0.9 to 1.5% by weight is at least 2% of the diameter ofthe rolling body. Incidentally, where the member made of the casehardened bearing steel is manufactured by the method describedpreviously, it is substantially impossible for the surface region havinga carbon concentration of 0.9 to 1.5% by weight to be formed in anexcessively large thickness. The largest thickness of the case hardenedlayer that can be formed is about 2 mm.

Some examples of the present invention will now be described.

Specifically, the ball-and-roller bearing 1 shown in FIG. 1 wasmanufactured as follows. In the first step, each of the alloys havingthe compositions as shown in Table 1 was molded to prepare moldedarticles of the shapes corresponding to the inner ring 2, the outer ring3 and the balls (rolling bodies) 4. Then, heat treatments similar tothose applied to a test piece, which are described herein later, wereapplied to these molded articles to prepare the inner ring 2, the outerring 3 and the balls 4. Further, these inner ring 2, outer ring 3 andballs 4 were assembled to manufacture the ball-and-roller bearing 1. Theball-and-roller bearings 1 thus manufactured are given in Table 1 assamples Nos. (1) to (21).

TABLE 1 Concentration in alloy (% by weight) Sample C Si Mn Cr Mo V CoNi  (1) 0.04 0.31 0.50 7.0 3.5 0.38 3.0 2.0  (2) 0.08 0.27 0.45 8.0 4.5— 3.5 —  (3) 0.04 0.30 1.50 9.0 3.9 0.42 4.8 1.5  (4) 0.07 0.28 0.27 9.04.9 0.10 5.1 1.3  (5) 0.03 0.28 0.31 9.1 5.1 0.12 4.9 1.2  (6) 0.04 0.200.51 9.2 6.0 0.11 2.6 4.0  (7) 0.08 0.24 0.28 9.0 3.2 — 2.1 1.3  (8)0.07 0.28 0.30 9.1 3.0 0.10 2.0 1.3  (9) 0.07 0.25 0.27 9.0 3.1 0.30 2.51.5 (10) 0.02 0.25 0.32 9.5 2.0 0.32 8.0 1.0 (11) 0.11 0.35 0.28 8.9 4.90.11 5.0 1.3 (12) 0.20 0.28 0.31 9.3 3.3 0.30 1.5 1.1 (13) 0.02 0.250.55 9.5 3.0 1.00 3.2 5.0 (14) 0.08 0.24 0.38 10.0  3.1 0.32 4.8 2.1(15) 0.06 0.30 0.35 11.0  3.2 0.43 3.5 2.5 (16) 0.08 0.28 0.32 12.8  3.10.11 4.2 1.7 (17) 0.07 0.27 0.32 6.0 3.0 0.32 3.2 1.5 (18) 0.07 0.250.27 9.0 3.1 0.30 2.5 1.5 (19) 0.07 0.25 0.27 9.0 3.1 0.30 2.5 1.5 (20)0.07 0.28 0.31 9.1 1.5 0.40 3.4 1.3 (21) 0.83 0.23 0.31 4.1 4.2 0.99 — —

In Table 1, the amounts of the elements added to iron are denoted by “%by weight” to the alloy. Samples (1) to (15) given in Table 1 representball-and-roller bearings according to the examples of the presentinvention. On the other hand, samples (16) to (20) representball-and-roller bearings for comparative examples. Further, sample (21)represents the conventional ball-and-roller bearing.

In order to examine the rolling fatigue life and the corrosionresistance of these samples (1) to (21), test pieces were prepared byusing the alloys of the compositions shown in Table 1, and heattreatments were applied to these test pieces under the conditions equalto those under which the heat treatments were applied to the samples (1)to (21). The test piece used for the rolling fatigue life test was sizedat 60 mm in outer diameter, 5.5 mm in inner diameter, and 6 mm inthickness. On the other hand, the test piece used for the corrosionresistance test was sized at 20 mm in outer diameter and 10 mm inthickness. A cutting operation was applied as a finishing treatment toeach of the test pieces after the heat treatments so as to obtain testpieces of a predetermined shape and size. Further, a lapping treatmentwas applied to the test surface of the test piece for the rollingfatigue life test.

Heat treatments were applied during preparation of the test pieces underthe conditions given below.

Specifically, the test pieces of the compositions equal to those of thealloys used for forming samples (1) to (20) were prepared by applyingheat treatments under the temperature conditions shown in FIG. 3, whichis a graph showing the heat treating conditions of the alloys used forforming the ball-and-roller bearings according to the examples of thepresent invention and comparatives examples. In the graph of FIG. 3, thetime (arbitrary unit) is plotted on the abscissa, with the heatingtemperature being plotted on the ordinate. As shown in the graph, thecase hardened bearing steel used for preparing samples (1) to (20) wassubjected to a cementating treatment under vacuum at 930 to 950° C.,followed by temporarily cooling the samples and subsequently applying ahardening treatment to the bearing steel at 1030 to 1080° C. Further, atempering treatment was applied at 500 to 600° C. for 2 hours to thebearing steel. The tempering treatment was applied three times so as toachieve a secondary hardening.

As shown in Table 1, the alloys used preparing samples (9), (18) and(19) were equal to each other in composition. However, these samples(9), (18) and (19) differed from each other in the carbon concentrationin the case hardened layer, as shown in Table 2. The carbonconcentration in the case hardened layer was controlled by controllingappropriately the time for the vacuum cementating treatment, the heatingtime before the hardening treatment, etc.

The test piece of the composition equal to that of the alloy used forpreparing sample (21) was hardened at 1130° C., followed by applying atempering treatment to the test piece at 550° C. for 2 hours, thetempering treatment being applied three times. Incidentally, aconventional alloy of AISI M50, which is widely used for preparing aball-and-roller bearing for an aircraft, was used for preparing sample(21).

The rolling fatigue life and the corrosion resistance were measured byusing the test pieces thus prepared for testing the rolling fatigue lifeand for testing the corrosion resistance. A testing machine shown inFIG. 4 was used for measuring the rolling fatigue life.

Specifically, FIG. 4 is a cross sectional view schematically showing athrust type life testing machine used for measuring the rolling fatiguelife of the ball-and-roller bearing 1 according to the examples of thepresent invention.

As shown in FIG. 4, the testing machine includes a housing 11. Atruncated cone-shaped recess having a bottom smaller than the upperopening is formed in an upper portion of the housing 11. Also, a testpiece 6 for testing the rolling fatigue life is fixed to the bottom ofthe recess. A rotary shaft 12 having an inner ring 7 fixed to the tip isarranged above the test piece 6. Balls 9 held by a holder 8 areinterposed between the inner ring 7 and the test piece 6. The balls 9are rolled along the upper surface of the test piece 6 in accordancewith rotation of the rotary shaft 12. It should be noted that the innerring 7 and the holder 8 shown in FIG. 4 are what are used for the lifetest and, thus, differ in construction from the inner ring and theholder actually used in a ball-and-roller bearing used in, for example,an aircraft. Also, the balls 9 were made of AISI M50. Where the balls 9were deteriorated during the test, new balls were promptly substitutedfor the deteriorated ball.

The recess of the housing 11 is filled with a lubricating oil 13, whichis supplied to the surfaces of the test piece 6, the inner ring 7, theballs 9 and the holder 8. A heater 14 is buried in a bottom portion ofthe housing 11 so as to heat the lubricating oil 13 through the housing11.

The thrust type life testing machine of the particular constructiondescribed above makes it possible to reproduce various conditions of useof the bearing by changing the rotating speed of the rotary shaft 12,the axial load applied from the rotary shaft 12 to the test piece 6, thetemperature of the lubricating oil, etc. In this experiment, a powderysteel was added as a foreign matter to the lubricating oil 13, and therolling fatigue test was conducted under the conditions given below:

Load applied to the test piece: P_(max)=5.5 GPa;

Rotary shaft: 1000 rpm;

Lubricating oil: R_(O)150 (142 cSt/40° C.);

Lubricating oil temperature: 130° C.

The rolling fatigue test was applied to 15 samples for each test piece.A Weibull plot was prepared by using as the life value the number ofrepetitions of the stress application until peeling occurred on thesurface of the test piece, and L₁₀ life of each of the test pieces wasobtained from the result of the Weibull distribution.

For conducting the corrosion resistance test, the test piece for thecorrosion resistance test was kept dipped for 24 hours in a city waterof room temperature, and the corrosion resistance was evaluated byobserving the rust occurrence on the test piece after dipping in thecity water.

The results of the rolling fatigue test and the corrosion resistancetest are shown in Table 2 together with the surface hardness of each ofthe test pieces.

TABLE 2 Carbon Rolling concen- fatigue tration in life case Surface(×10⁶ hardened hardness Stress Corrosion Sample layer (wt %) (HRC)Cycle) resistance  (1) 1.02 64.2 8.9 ◯  (2) 0.95 65.1 9.2 ◯  (3) 1.1665.0 10.1  ◯  (4) 1.45 65.5 11.0  ◯  (5) 1.11 65.7 10.3  ◯  (6) 0.9066.8 12.1  ◯  (7) 1.50 64.2 9.7 ◯  (8) 1.35 64.3 9.5 ◯  (9) 1.27 64.59.8 ◯ (10) 1.07 64.1 9.3 ◯ (11) 1.24 66.2 11.8  ◯ (12) 1.33 64.3 9.1 ◯(13) 1.41 64.2 9.5 ◯ (14) 1.22 64.5 9.6 ◯ (15) 1.36 64.8 10.2  ◯ (16)1.42 64.6 3.6 ◯ (17) 1.33 64.2 8.5 X (18) 0.79 62.2 2.4 ◯ (19) 1.78 64.83.8 ◯ (20) 1.31 61.8 2.1 ◯ (21) 0.76 62.7 2.4 X

Mark “X” shown in Table 2 denotes that rust was recognized in thesample. Also, mark “◯” represents that rust was not recognized in thesample. The samples marked “◯” are considered to exhibit corrosionresistance equal to or higher than that of the sample prepared by usingSUS440C, which is a conventional martensite series stainless steel.

As shown in Table 2, samples (18), (20) and (21) were found to be low inthe surface hardness. On the other hand, samples (1) to (15) accordingto the examples of the present invention exhibited a high surfacehardness, i.e., HRC of 64 or more.

As described previously, samples (1) to (15) were obtained by applying ahardening treatment at a low temperature, i.e., 1100° C. or lower, and asecondary hardening treatment at 500° C. or higher. On the other hand,in order to obtain a sufficiently high surface hardness by a secondaryhardening treatment in the case of using the conventional semihiceseries material, it is necessary to apply a hardening treatment at ahigh temperature exceeding 1100° C. It should also be noted that thesurface hardness is lowered, if a tempering treatment is carried out ata high temperature exceeding 450° C. in the case of using a stainlesssteel series material. As apparent from the experimental data given inTable 2, in samples (1) to (15) according to the examples of the presentinvention, the hardening treatment can be carried out at a temperaturelower than that in the comparative examples and the sample of theconventional material. In addition, it is possible to obtain a surfacehardness higher than that of the comparative examples and the sample ofthe conventional material.

When it comes to the rolling fatigue life, the L₁₀ life for each ofsamples (1) to (15) was found to be longer than that of any of samples(16) to (21). Particularly, samples (1) to (15) according to theexamples of the present invention were found to exhibit a rollingfatigue life at least 3 times as long as that of any of samples (16) and(18) to (20) according to the comparative examples, and sample (21) ofthe conventional material. What should also be noted is that aninconvenience such as crack occurrence, which is derived from aninsufficient toughness of the core portion, was not recognized at all insamples (1) to (15) during the rolling fatigue life test.

The samples according to the examples of the present invention were alsosatisfactory in the results of the corrosion resistance test.Specifically, the rust occurrence was not recognized at all in any ofsamples (1) to (15). On the other hand, rust was generated in samples(17) and (21), indicating that these samples were inferior to the othersamples in the corrosion resistance. It should be noted in thisconnection that the chromium concentration in each of samples (17) and(21) was lower than 7% by weight, leading to the rust generation.

To reiterate, the hardening temperature for each of samples (1) to (15)was relatively low. In addition, these samples were found to beexcellent in the mechanical strength such as the rolling fatigue lifecharacteristics and to exhibit a high corrosion resistance. In otherwords, samples (1) to (15), which represent the ball-and-roller bearing1 according to the examples of the present invention, were found toexhibit excellent life characteristics.

The carbon concentration in a surface region of each of samples (1) to(21) is also given in Table 2. The term “surface region” represents aregion having a depth, as measured from the surface of each sample,equal to 2% of the diameter of the ball 4. Also, the carbonconcentration in the surface region was measured by an emissionspectroscopic analysis by dissolving the surface region of each samplein a solvent.

For example, comparing samples (9) and (18), these samples are differentin the carbon concentration in the surface region, i.e., the carbonconcentration in the surface region of the former falls within a rangeof between 0.9 and 1.5% by weight, while that of the latter is less than0.9% by weight. It should also be noted that each of samples (1) to (15)according to the examples of the present invention exhibits a surfacehardness higher than that of sample (18) and a L₁₀ life longer than thatof sample (18). The experimental data clearly support that asufficiently high surface hardness can be obtained by setting the carbonconcentration in the surface region at 0.9% by weight or more.

Likewise, samples (9) and (19) are different in the carbon concentrationin the surface region, i.e., that of former falls within a range ofbetween 0.9 and 1.5% by weight while the latter higher than 1.5% byweight. Sample (19) is certainly substantially equal in its surfacehardness to samples (1) to (15). However, sample (19) is markedlyinferior in its L₁₀ life to samples (1) to (15). It should be noted inthis connection that, since the carbon concentration in the surfaceregion exceeds 1.5% by weight in sample (19), a coarse mesh-like carbidestructure is formed in the surface region so as to give rise to the poorL₁₀ life. To be more specific, the mesh-like carbide structure acts asan origin of crack occurrence so as to have markedly shortened the lifeof sample (19).

It should be noted that sample (16) falls within the scope defined inthe present invention, except that the chromium concentration alonefails to fall within the range specified in the present invention. To bemore specific, the chromium concentration in any of samples (1) to (15)according to the embodiment of the present invention falls within arange of between 7.0 and 11.0% by weight, whereas, the chromiumconcentration of sample (16) is 12.8% by weight, which exceeds the upperlimit of 11.0% by weight specified in the present invention. As result,sample (16), which certainly exhibits a sufficiently high surfacehardness, tends to incur seizing and has a rolling fatigue life shorterthan that of any of samples (1) to (15).

The relationship between the surface hardness and the molybdenumconcentration in the case hardened bearing steel and the relationshipbetween the surface hardness and the cobalt concentration in the casehardened bearing steel were examined by the method described in thefollowing.

Specifically, the relationship between the cobalt concentration andsurface hardness was examined first by changing the cobalt concentrationin the case hardened bearing steel, with the concentrations of the othercomponents set constant. FIG. 5 is a graph showing the result. Then, therelationship between the molybdenum concentration and surface hardnesswas examined by changing the molybdenum concentration in the casehardened bearing steel, with the concentrations of the other componentsset constant. FIG. 6 is a graph showing the result.

In the graph shown in FIG. 5, the cobalt concentration is plotted on theabscissa, with the surface hardness (HRC) being plotted on the ordinate.Also, in the graph shown in FIG. 6, the molybdenum concentration isplotted on the abscissa, with the surface hardness (HRC) being plottedon the ordinate.

A curve 20 shown in FIG. 5 covers the case where the case hardenedbearing steel contained 5 to 6% by weight of molybdenum, 0.2 to 0.4% byweight of silicon, 0.2 to 0.5% by weight of manganese, 7 to 8% by weightof chromium, 0 to 0.1% by weight of vanadium, 0.5 to 1.5% by weight ofnickel, and varied amounts of cobalt, and where the carbon concentrationin the surface region of the case hardened bearing steel was set at 0.9to 1.5% by weight. A curve 21 shown in FIG. 5 covers the casesubstantially equal to the case denoted by the curve 20, except that thecase hardened bearing steel contained 2 to 3% by weight of molybdenumand 10 to 11% by weight of chromium. Further, a curve 22 shown in FIG. 5covers the case substantially equal to the case denoted by the curve 21,except that the case hardened bearing steel contained 1% by weight ofmolybdenum.

A curve 23 shown in FIG. 6 covers the case where the case hardenedbearing steel contained 5 to 6% by weight of molybdenum, 7 to 8% byweight of cobalt, 0.3 to 0.4% by weight of silicon, 0.3 to 0.5% byweight of manganese, 10 to 11% by weight of chromium, 0.1 to 0.2% byweight of vanadium, 1 to 2% by weight of nickel, and varied amounts ofmolybdenum, and where the carbon concentration in the surface region ofthe case hardened bearing steel was set at 0.9 to 1.5% by weight. Acurve 24 shown in FIG. 6 covers the case substantially equal to the casedenoted by the curve 23, except that the case hardened bearing steelcontained 2 to 3% by weight of cobalt and 7 to 8% by weight of chromium.Further, a curve 25 shown in FIG. 6 covers the case substantially equalto the case denoted by the curve 23, except that the case hardenedbearing steel contained 0% by weight of cobalt and 9 to 10% by weight ofchromium.

In a ball-and-roller bearing used under a high temperature and at a highrotating speed like a ball-and-roller bearing used for supporting arotary shaft in an engine of an aircraft, a very high demand is directedto the rolling fatigue life characteristics. In general, the rollingfatigue life of a ball-and-roller bearing is deeply related to thesurface hardness of the bearing member. In order to obtain satisfactoryrolling fatigue life characteristics in a ball-and-roller bearing usedin, for example, an engine of an aircraft, it is said that the surfacehardness (HRC) of at least 63, preferably at least 64, is required.

Where the molybdenum concentration in the case hardened bearing steel isset at 1% by weight, it is impossible to obtain the surface hardness(HRC) of at least 63 by changing the cobalt concentration, as shown inFIG. 5. On the other hand, where the molybdenum concentration is set at2% by weight or more, the surface hardness (HRC) is increased withincrease in the cobalt concentration.

As shown in FIG. 5, where the case hardened bearing steel has acomposition described previously in conjunction with curve 20, thesurface hardness (HRC) can be made 63 or more by setting the cobaltconcentration at 0.5% by weight or more and can be made 64 or more bysetting the cobalt concentration at 1.0% by weight or more. On the otherhand, where the case hardened bearing steel has a composition describedpreviously in conjunction with curve 21, the surface hardness (HRC) canbe made 63 or more by setting the cobalt concentration at 0.7% by weightor more and can be made 64 or more by setting the cobalt concentrationat 1.5% by weight or more.

Comparing curves 20 to 22 in FIG. 5, it is clearly shown that curves 20and 21 widely differ from curve 22 in the surface hardness. However, alarge difference in the surface hardness is not recognized betweencurves 20 and 21. In other words, it is impossible to improve markedlythe surface hardness by increasing the molybdenum concentration in thecase hardened bearing steel to exceed 6.0% by weight. It should also benoted that molybdenum is relatively costly. In addition, the toughnessis adversely affected in the case of excessively adding molybdenum. Itfollows that the increase in the manufacturing cost can be suppressedand the reduction in the toughness can be prevented by setting themolybdenum concentration in the case hardened bearing steel at 6.0% byweight or less.

Next, curves 23 to 25 shown in FIG. 6 are compared. As is apparent fromthe curves 23 to 25, the surface hardness can be increased by increasingthe molybdenum concentration in the case hardened bearing steel.However, where the case hardened bearing steel does not contain cobalt,it is impossible to obtain a sufficiently high surface hardness even ifthe molybdenum concentration is increased up to 6% by weight. On theother hand, where the cobalt concentration in the case hardened bearingsteel is set at 2% by weight or more, the surface hardness can bemarkedly improved by the addition of only a small amount of molybdenum.

As shown in FIG. 6, where the case hardened bearing steel has acomposition described previously in conjunction with curve 23, thesurface hardness (HRC) can be made 63 or more by setting the molybdenumconcentration at 1.5% by weight or more and can be made 64 or more bysetting the molybdenum concentration at 1.7% by weight or more. On theother hand, where the case hardened bearing steel has a compositiondescribed previously in conjunction with curve 24, the surface hardness(HRC) can be made 63 or more by setting the molybdenum concentration at1.7% by weight or more and can be made 64 or more by setting the cobaltconcentration at 2.0% by weight or more.

Also, where the cobalt concentration in the case hardened bearing steelis set at 2% by weight or more, the surface hardness (HRC) of 64 or morecan be obtained with a high stability by setting the molybdenumconcentration at 2% by weight or more, as apparent from FIG. 6.

Comparing curves 23 to 25 in FIG. 6, it is clearly shown that curves 23and 24 widely differ from curve 25 in the surface hardness. However, alarge difference in the surface hardness is not recognized betweencurves 23 and 24. In other words, it is impossible to improve markedlythe surface hardness by increasing the cobalt concentration in the casehardened bearing steel to exceed 8.0% by weight. It should also be notedthat cobalt is relatively costly. In addition, the toughness isadversely affected in the case of excessively adding cobalt. It followsthat the increase in the manufacturing cost can be suppressed and thereduction in the toughness can be prevented by setting the cobaltconcentration in the case hardened bearing steel at 8.0% by weight orless.

The relationship among the molybdenum concentration and the cobaltconcentration in the case hardened bearing steel and the surfacehardness was studied on the basis of the experimental data given inFIGS. 5 and 6, with the result as shown in a graph of FIG. 7. In thegraph of FIG. 7, the molybdenum concentration is plotted on theabscissa, with the cobalt concentration being plotted on the ordinate.It should be noted that the concentrations of the other componentsconstituting the case hardened bearing steel fell within the rangesdefined in the present invention.

In the graph of FIG. 7, the region surrounded by A-B-C-D denotes a rangewithin which the surface hardness (HRC) of at least 63 can be obtained.The region surrounded by E-F-G-D denotes a range within which thesurface hardness (HRC) of at least 64 can be obtained. Further, theregion surrounded by H-I-J-D denotes a range within which the surfacehardness (HRC) of a further higher level can be obtained. It followsthat the molybdenum concentration and the cobalt concentration requiredfor obtaining a sufficiently high surface hardness can be determined byusing the graph of FIG. 7.

As described above, any of the ball-and-roller bearings according to theexamples of the present invention exhibits a high corrosion resistanceand excellent rolling fatigue life characteristics. It follows that eachof the ball-and-roller bearings according to the examples of the presentinvention is suitable for use as a ball-and-roller bearing used underthe conditions of a high temperature and a high rotating speed like aball-and-roller bearing used in an engine of an aircraft. For example,the ball-and-roller bearings according to the examples of the presentinvention can be used even where the dN value is set at about 3,000,000to 4,000,000 and the temperature is set at about 300° C. to 400° C.Incidentally, the term “dN value” represents the product between theinner diameter (mm) of the inner ring and the number of revolutions perunit time (rpm).

In the examples described above, a cementating treatment was applied tothe alloy under vacuum. However, since the cementating treatment isintended to supply carbon required for the secondary hardening to thesurface of the alloy, the cementating treatment is not limited to thevacuum cementating treatment. Specifically, the similar effect can beobtained by, for example, a plasma cementating treatment or acarbonitriding treatment.

Also, in the examples described above, a case hardened bearing steel ofa predetermined composition is used for forming all of the inner ring,the outer ring, and the rolling bodies. However, it is not absolutelynecessary to use the case hardened bearing steel specified in thepresent invention for forming all of the inner ring, the outer ring, andthe rolling bodies. It should be noted that the rolling body is lesslikely to be exposed to the outer atmosphere than the inner ring and theouter ring and has a size and a surface area smaller than those of theinner ring and the outer ring. It follows that the corrosion problem ofthe rolling bodies are not so serious. Also, since the rolling body issized small, the requirement for a high toughness in the inner portionis relatively low in the rolling body. It follows that it is possible touse, for example, a semihice series materials of AISI M50 or M50 NiL forforming the rolling bodies.

It is also possible to use, for example, a ceramic material such assilicon nitride, alumina, or zirconia for forming the rolling bodies.Where the rolling bodies are formed of such a ceramic material, it ispossible to improve the heat resistance, the surface hardness and thecorrosion resistance.

The mechanical characteristics and the corrosion resistance can beimproved even where one or two of the inner ring, the outer ring and therolling body is formed of the case hardened bearing steel specified inthe present invention while forming the remaining member by usinganother material.

As described above, the present invention provides a ball-and-rollerbearing, in which at least one of the inner ring, the outer ring, andthe rolling body is formed of a case hardened bearing steel consistingof an alloy of a predetermined composition and having a case hardenedlayer. It follows that the present invention provides a ball-and-rollerbearing excellent in mechanical characteristics and corrosion resistanceand a method of manufacturing the same.

What is claimed is:
 1. A ball-and-roller bearing which exhibits anexcellent corrosion resistance, comprising: an inner ring; an outer ringarranged on the co-axis of the inner ring and rotating around the axisrelative to the inner ring; and a rolling body interposed between theinner ring and the outer ring and rolling on the inner ring and theouter ring in accordance with rotation of the outer ring relative to theinner ring, wherein at least one member selected from the groupconsisting of the inner ring, the outer ring, and the rolling bodycomprises a core member consisting essentially of an alloy consistingessentially of iron, 0.15% by weight or less of carbon, at least one of0.2 to 1.0% by weight of silicon and 0.2 to 1.5% by weight of manganese,7.0 to 11.0% by weight of chromium, 1.5 to 6.0% by weight of molybdenum,0.5 to 8.0% by weight of cobalt, 1.0% by weight or less of vanadium, and5.0% by weight or less of nickel, and a case hardened layer formed bysubjecting a surface region of said core member to a secondary hardeningtreatment and containing 0.9 to 1.5% by weight of carbon.
 2. Theball-and-roller bearing according to claim 1, wherein said alloycontains 0.1 to 1.0% by weight of vanadium and 1.0 to 5.0% by weight ofnickel.
 3. A method of manufacturing a ball-and-roller bearing whichexhibits an excellent corrosion resistance, comprising the steps of:forming a core member consisting essentially of an alloy consistingessentially of iron, 0.15% by weight or less of carbon, at least one of0.2 to 1.0% by weight of silicon and 0.2 to 1.5% by weight of manganese,7.0 to 11.0% by weight of chromium, 1.5 to 6.0% by weight of molybdenum,0.5 to 8.0% by weight of cobalt, 1.0% by weight or less of vanadium, and5.0% by weight or less of nickel; forming a case hardened layercontaining 0.9 to 1.5% by weight of carbon in a surface region of saidcore member by applying to said core member a carbonizing orcarbonitriding treatment, a hardening treatment, and a temperingtreatment under high temperatures in the order mentioned; and assemblingan inner ring, an outer ring and a rolling body such that said innerring and said outer ring are positioned on the same axis and that saidrolling body is interposed between said inner ring and said outer ring,at least one of said inner ring, said outer ring, and said rolling bodybeing formed of said core member having said case hardened layer formedon the surface.
 4. The method of manufacturing a ball-and-roller bearingaccording to claim 3, wherein said alloy comprises 0.1 to 1.0% by weightof vanadium and 1.0 to 5.0% by weight of nickel.
 5. The ball-and-rollerbearing according to claim 1, wherein said core member containsmolybdenum and cobalt in a concentration range surrounded by solid linesA-B, B-C, C-D, and D-A shown in FIG.
 7. 6. The ball-and-roller bearingaccording to claim 2, wherein said core member contains molybdenum andcobalt in a concentration range surrounded by solid lines A-B, B-C, C-D,and D-A shown in FIG.
 7. 7. The ball-and-roller bearing according toclaim 1, wherein said alloy contains 8.0 to 11.0% by weight of chromium.8. The method of manufacturing a ball-and-roller bearing according toclaim 3, wherein said core member contains molybdenum and cobalt in aconcentration range surrounded by solid lines A-B, B-C, C-D, and D-Ashown in FIG.
 7. 9. The method of manufacturing a ball-and-rollerbearing according to claim 4, wherein said core member containsmolybdenum and cobalt in a concentration range surrounded by solid linesA-B, B-C, C-D, and D-A shown in FIG.
 7. 10. The method of manufacturinga ball-and-roller bearing according to claim 3, wherein said alloycontains 8.0 to 11.0% by weight of chromium.
 11. A ball-and-rollerbearing which exhibits an excellent corrosion resistance, comprising: aninner ring; an outer ring arranged on the co-axis of the inner ring androtating around the axis relative to the inner ring; and a rolling bodyinterposed between the inner ring and the outer ring and rolling on theinner ring and the outer ring in accordance with rotation of the outerring relative to the inner ring, wherein at least one member selectedfrom the group consisting of the inner ring, the outer ring, and therolling body comprises a core member of an alloy comprising of iron,about 0 to 0.15% by weight of carbon, at least one of 0.2 to 1.0% byweight of silicon and 0.2 to 1.5% by weight of manganese, 7.0 to 11.0%by weight of chromium, 1.5 to 6.0% by weight of molybdenum, 0.5 to 8.0%by weight of cobalt, and 1.0 to 5.0% by weight of nickel, and a casehardened layer formed by subjecting a surface region of said core memberto a secondary hardening treatment and containing 0.9 to 1.5% by weightof carbon.